Upon drilling crude oil, bitumen, a natural gas or the like, together with the production of oil or gas, ground water intermingled therewith is discharged as produced water. Besides, when water (steam) is injected into an oil field or a gas field for drilling the crude oil or the like, the water may also be discharged as produced water. Such produced water may contain various kinds of chemical compositions or oil according to the features of a stratum or an injected chemical. Therefore, it has been generally implemented that these concentrations are reduced by applying water treatment prior to discharging it to the outside or, alternatively, re-injecting it into the oil field, the gas field, an aquifer or the like. Above all, in a water shortage area, water can hardly be secured in a sufficient amount to be used for water flooding. As such, it has been more frequent that the produced water is treated to be used as injected water. With regard to discharging the produced water into the sea or rivers or reinjection, regulations are particularly reinforced in recent years in response to the rising consciousness of environmental issues. Thus, the development of advanced water treatment technology is highly expected.
With regard to the treatment of the produced water (oil-containing water), a conventional technique has been applied as below exemplified (see FIG. 10). Crude oil fluid mixture 20A drilled from underground (oil well) 1 is sent to HP separator 2. Crude oil-containing liquid treated herein is sent in feed direction d1, and further treated by means of LP separator 3. The liquid is then cooled by means of heat exchanger 51b to the extent in which the treated liquid is not boiled. The resulting liquid is further treated by means of dehydrator 4, and shipped as crude oil (product) (in the drawing, a butterfly-shaped part in which two isosceles triangles are combined represents a valve).
Apart therefrom, oil-containing produced water (referred to as oil-containing water, produced water or the like in several cases) 20B separated by means of HP separator 2 is sent in a feed direction d2, cooled to a predetermined temperature by means of heat exchanger 51a, and then once stored in skim tank 5. At this time, part of produced water is separated also from LP separator 3, and mixed with produced water from the HP separator. An evaporative gas is also generated in HP separator 2 and LP separator 3, and it is also discharged to the outside, respectively. Subsequently, the oil is separated and removed in a flow of induced gas flotation 6 and oil removal filter 7 using walnut shells or the like, and conventional treated water 20D′ is recovered (defined as 20D′ so as to be distinguished from treated water 20D of the present invention as described later). Oil-water separation in this method is achieved basically by gravity separation utilizing a difference in specific gravity between oil and water.
As a specific technology development, Patent Literature 1 discloses an example of separating and removing oil from water which is brought with crude oil upon drilling it from a submarine oil field. This literature also discloses an example of applying microbial treatment to water which is obtained by oil-water separation through induced emulsion breaking.